1. Field of the Invention
This invention relates to optical fibers and more particularly, to infrared ray-transmittable fibers. The optical fibers of the invention are particularly suitable for energy transmission such as in laser works and laser scalpels.
2. Description of the Prior Art
In recent years, attempts have been made to utilize laser beams in various fields. For instance, carbon dioxide laser scalpels have been progressively developed in medical fields and effectively utilized. In the field of industrial machinery and tools, efforts have been made to apply laser beams to processing machines for cutting, marking, and trimming.
Aside from carbon dioxide lasers having an oscillation wavelength of 10.6 .mu.m, other infrared lasers such as a carbon monoxide laser having an oscillation wavelength of 5 to 7 .mu.m are also used. In order to effectively utilize these infrared laser beams having high power energy, there is a high demand for optical fibers having good flexibility and capable of transmitting high energy power.
Optical fibers for moderate infrared radiation were developed using various materials such as chalcogen compounds, fluorides, metal halides and the like. However, chalcogen compounds and fluorides have drawbacks: chalcogen compounds are not suitable for high power energy transmission; and fluoride compounds involve the problem that infrared beams attenuate at wavelengths over 7 .mu.m. On the other hand, metal halides have a great possibility for use as an infrared fiber. For instance, there are known materials, called KRS-5, which are frequently used as optical parts for infrared radiation. These materials are permeable to radiation in the moderate infrared region covering a wavelength of from 0.5 to 30 .mu.m, and have fairly good humidity resistance and mechanical strength. The KRS-5 materials are solid solutions of thallium bromide, TlBr, and thallium iodide, TlI, typical of which is a composition comprising 38 wt% of thallium bromide.
These materials are converted to fibers by a procedure which comprises heating the material at temperatures 100.degree. to 200.degree. C. lower than the melting point of the solid solution, KRS-5, and extruding under high pressure. Aside from the solid solutions, thallium chloride, thallium bromide, and thallium iodide are singly used for conversion to a fiber. In practice, the optical fibers obtained from these thallium halides have a diameter of from 0.3 to 1 mm. However, when high power over 30 W is transmitted to the optical fiber, the fiber tends to deteriorate within a relatively short period of time, and may be readily melted during use. Thus, high power transmission becomes impossible. We found that the deterioration was attributed to impurities such as metal oxides and chlorides and also to the size of the impurities contained in the starting thallium halide materials.